How to identify Weathered Iron-Rich Sediment

To correctly identify Weathered Iron-Rich Sediment (Oxidized Sedimentary Material), check each of these features in order. Many rocks and minerals look alike, so cross-reference multiple properties before deciding.

Step-by-step identification

1. 1

   Color

   Overall color and any zoning or banding

   Predominantly reddish-brown, orange, yellow, or deep red. The specific hue depends on the dominant iron oxide/oxyhydroxide mineral (e.g., hematite for red, goethite for yellow/brown).

2. 2

   Luster

   How the surface reflects light: metallic, vitreous, dull

   Dull to earthy, sometimes slightly vitreous if quartz grains are present.

3. 3

   Texture

   Grain size and surface feel: coarse, fine, glassy

   Variable, ranging from fine-grained (clay, silt) to coarse-grained (sand, gravel). Often friable and unconsolidated, but can be partially cemented or indurated.

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

   Individual mineral grains are typically anhedral (irregularly shaped). Iron oxides/oxyhydroxides often occur as amorphous coatings or microcrystalline aggregates.

5. 5

   Cleavage

   How it breaks: flat cleavage planes vs irregular fracture

   Not applicable to the bulk sediment; individual mineral components may exhibit cleavage (e.g., clay minerals), but it's not a characteristic of the weathered sediment itself.

6. 6

   Geological Environment

   The rock formation and setting where it occurs

   Common in terrestrial environments, particularly in tropical and subtropical regions with intense chemical weathering. Found in soils, regolith, fluvial deposits, alluvial fans, and ancient paleosols. Also present in some marine and lacustrine settings where iron-rich detritus is deposited and subsequently oxidized.

Key Facts

Hardness

Variable, typically very soft (1-3 on Mohs scale) for individual iron oxyhydroxide minerals, but the bulk sediment can be harder if cemented.

Specific Gravity

Variable, typically 2.7-3.5, depending on the proportion of iron oxides/oxyhydroxides and other minerals.

Crystal System

Not applicable to the bulk sediment. Individual iron oxides/oxyhydroxides can be amorphous (ferrihydrite), monoclinic (goethite), or trigonal/hexagonal (hematite).

Color

Reddish-brown, orange, yellow, deep red.

Luster

Dull to earthy.

Transparency

Opaque.

Fracture

Not applicable to the bulk sediment; individual grains may show conchoidal or irregular fracture.

Cleavage

Not applicable to the bulk sediment.

Composition

Primarily iron oxyhydroxides (goethite, ferrihydrite) and iron oxides (hematite), mixed with varying amounts of quartz, clay minerals (kaolinite, illite), and other resistant detrital minerals. Minor amounts of manganese oxides may also be present.

Physical characteristics

• Crystal Habit: Not applicable to the bulk sediment. Iron oxides/oxyhydroxides often occur as cryptocrystalline or amorphous coatings and aggregates.
• Cleavage Type: Not applicable to the bulk sediment.
• Fracture Type: Not applicable to the bulk sediment; individual grains may show irregular or conchoidal fracture.
• Tenacity: Friable to brittle, depending on the degree of cementation.
• Luster Type: Earthy to dull.

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